<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <!-- 页面资源是9000来的, 用户数据也是9000来的, 二者同源 -->
    <h1>跨域</h1>
    <button>点击获取用户数据</button>
    <h1>该文件要用localhost:9000/home打开</h1>

    <script>
        const btn = document.querySelector('button');

        btn.onclick = function () {
            const xhr = new XMLHttpRequest();
            // 满足同源, url可以简写
            xhr.open('GET', '/data');
            xhr.send();
            xhr.onreadystatechange = function () {
                if (xhr.readyState === 4) {
                    if (xhr.status >= 200 && xhr.status < 300) {
                        console.log(xhr.response);
                    }
                }
            };
        };
    </script>
</body>
</html>